Due to the exhaustion of traditional energy sources together with an increase in the global energy demand, impetus is currently being given to the development of alternative energy sources. Among them, biomass is renewable quantitative biological resource that attracts a great deal of attention.
Among biomass-based industrial raw materials, isosorbide (C6H10O4) that is prepared by dehydration of sorbitol (C6H14O6) attracts attention as an environmentally friendly raw material for preparing polycarbonate (PC) as a substitute for bisphenol A (BPA), an epoxy monomer or an environmentally friendly plasticizer. Namely, isosorbide, a material that can be obtained by simple dehydration of sorbitol, is attracting attention as a monomer required for synthesis of next-generation, high-performance, environmentally friendly materials that can replace conventional polymer products, and many studies thereon have been conducted.
Environmentally friendly materials generally show poor properties compared to petrochemical-based materials, whereas isosorbide advantages in that it is environmentally friendly and, at the same time, shows excellent properties compared to conventional petrochemical-based materials. In addition, isosorbide serving as an agent for treating cardiac diseases may also be used as an additive that can make plastic materials stronger and tougher.
When D-glucose obtained from biomass by pretreatment is hydrogenated in the presence of a catalyst, sorbitol is produced. Isosorbide is produced by double dehydration of sorbitol. This cyclization reaction is influenced by various reaction conditions, including temperature, pressure, solvent, catalyst, etc.
Currently, as a method of preparing isosorbide from sorbitol, a process (Roquette process (France): G. Fleche, M. H. Lestrem, starch/starke 1986, 38, 26-30) is widely used in which sulfuric acid is used as a catalyst and a reaction is carried out under a reduced pressure of about 10 mmHg. However, when a liquid strong acid catalyst such as sulfuric acid is used, a reactor is easily corroded, and for this reason, an expensive reactor should be used. In addition, an additional process such as pH neutralization is required, and it is difficult to treat waste. Furthermore, a large amount of energy is continuously consumed to maintain a high vacuum level of about 10 mmHg, and thus the operating cost for the reaction is high. For this reason, a method employing molten salt hydrate (ChemSusChem. 2010, 3, 325-328) and the like have recently been proposed. However, this preparation method has problems in that, because molten salt hydrate should be used in very large amounts compared to the reactant sorbitol, the method is cost-ineffective and is not easy to commercialize.
Thus, if an efficient method for preparing and separating isosorbide is developed and a mass production process based on this method is provided so that a sufficiently inexpensive raw material (isosorbide) can be obtained, the demand for isosorbide as an industrial product can be increased.
Accordingly, the present inventors have found that, when the temperature of a high-pressure reaction that converts sorbitol to isosorbide is controlled in two steps, that is, a first low-temperature reaction step and a second high-temperature reaction step, in view of the fact that the rate of a latter-step reaction that converts the intermediate product 1,4-sorbitan (C6H12O5; 1,4-anhydro-sorbitol or 1,4-AHSO) to isosorbide is lower than the rate of a former-step reaction that converts sorbitol to 1,4-sorbitan in a high-pressure reaction that converts sorbitol to isosorbide, the yield of isosorbide in the high-pressure reaction can be economically increased even in the absence of a catalyst or in the presence of a small amount of a transition metal salt catalyst, thereby completing the present invention.